Home & Construction⏱ 5 min read
How to Calculate the Right Radiator Size for Any Room
An undersized radiator will never heat the room; oversized wastes energy. Here's how to calculate the BTU or watt output you need using room volume, heat loss, and insulation factors.
Radiator sizing is more precise than most DIY guides suggest. A rule of thumb based only on room area ignores ceiling height, window area, insulation quality, and room exposure — all of which have significant effects on heat loss.
The Core Calculation: Heat Loss
Required output (W) = Room volume (m³) × Heat loss factor (W/m³)
Room volume = Length × Width × Ceiling height
Heat loss factor varies by insulation quality:
Poor insulation (pre-1980s, single glazing): 60–80 W/m³
Average insulation (cavity walls, double glazing): 40–60 W/m³
Good insulation (modern, well-insulated): 25–40 W/m³
Excellent insulation (Passivhaus standard): 15–25 W/m³
Worked Example
Living room: 5m × 4m × 2.4m ceiling
= 48 m³
Semi-detached 1980s house, double glazing, cavity walls:
Heat loss factor: 50 W/m³ (average)
Required output = 48 × 50 = 2,400W
One standard double-panel radiator (600mm × 1200mm)
provides approximately 2,500–3,000W at 50°C ΔT ✓
Room Adjustment Factors
FactorAdjustment
External / north-facing room+10–15%
Large window area (>30% of wall)+10%
Poor loft insulation+15%
Solid stone / uninsulated walls+25–35%
Conservatory or extension+15–20%
Corner room (two external walls)+10%
BTU vs Watts Conversion
1 BTU/hr = 0.2931 Watts
1 Watt = 3.412 BTU/hr
2,400W × 3.412 = 8,189 BTU/hr
(Round up to nearest radiator size)
Standard Radiator Output Reference
Radiator Size (mm)TypeOutput at ΔT50 (approx)
400 × 600Single panel (K1)~500W
600 × 600Single panel (K1)~700W
600 × 1000Double panel (K2)~1,800W
600 × 1200Double panel (K2)~2,200W
600 × 1600Double panel (K2)~2,900W
600 × 2000Double panel (K2)~3,600W
Heat Pump Compatibility Note
Radiator output figures are quoted at ΔT50 — meaning the radiator flow temperature is 70°C and the room is 20°C. Heat pumps typically run at 35–55°C flow temperatures. At ΔT30 (45°C flow temperature), a radiator produces approximately 55% of its ΔT50 rating. If installing a heat pump, size radiators generously — typically 1.5–2× the calculated requirement — or use underfloor heating.